Currently, water-absorbent resins produced mainly from hydrophilic fiber such as pulp and acrylic acid (salt) are widely utilized as absorbents for sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads. From the viewpoint of improvement in recent QOL (quality of life), demand for such sanitary materials are shifting to those of lighter weight or of smaller thickness, and following this tendency, reduction of hydrophilic fiber usage has been demanded. Therefore, water-absorbent resin itself has been demanded to play the role of liquid diffusibility or initial absorption in an absorbent which have been taken by hydrophilic fibers, and water-absorbent resins superior in both liquid absorption under load and liquid permeability between swollen gels have been needed.
As a method for improving liquid permeability between swollen gels, there have been known (1) a method of forming physical spaces by adding an inorganic compound such as silica and talc to a surface of a water-absorbent resin, (2) a method of forming gel gaps by suppressing adhesion of swollen gels by treating a surface of a water-absorbent resin with a hydrophobic polymer small in surface free energy, such as modified silicone, and (3) a method of adding aluminum sulfate, aluminum lactate, etc. to a surface of a water-absorbent resin (see, for example, Patent Document 1, Patent Document 2, and Patent Document 3). However, the liquid permeability between swollen gels achieved by these methods are not fully satisfactory.
As another technique to improve liquid permeability between swollen gels, a technique of coating the surface of water-absorbent resin particles with a water-soluble polyamine, such as polyethyleneimine and polyvinylamine, has already been known (see, for example, Patent Document 4). However, the technique disclosed in Patent Document 4 allowed resin particles to adhere each other during coating, so that it was impossible to perform uniform coating. Therefore, it is impossible to sufficiently improve liquid permeability between swollen gels and, in addition, there is a problem that blocking or discoloration due to moisture absorption during storage is prone to occur and an unpleasant odor is generated.